1. Field of the Invention
The present invention relates to a refrigeration unit that includes a variable performance compressor.
2. Description of the Related Art
A refrigeration unit having a variable performance compressor is commonly used for a refrigerator, a freezer, a vending machine, an ice maker, an air conditioner or the like, and the basic construction thereof is as follows. A compressor driven by an inverter motor, a condenser with a cooling fan, a throttle valve such as a capillary tube, and an evaporator, for example, are sequentially connected by a refrigerant circuit, in which a refrigerant is compressed by the compressor and thereafter cooled through the condenser, so that a cooling action is performed through the evaporator by vaporizing the refrigerant.
Generally, in the refrigeration unit of this type, the devices on the refrigerant circuit may be damaged if the pressure in the refrigerant circuit increases too much. Therefore, as shown in JP-B-H06-3323, for example, the refrigerant pressure is detected directly or indirectly, and a protective operation, which halts the compressor or lowers the performance thereof to a predetermined level, is performed immediately after the detected pressure exceeds a predetermined limit value.
However, a problem arises that a refrigeration unit is prone to perform the above protective operation at short intervals when it is used in a harsh environment (e.g., for industrial use).
In an industrial refrigerator installed in the kitchen of a restaurant, for example, its doors are frequently opened at lunch or dinner time and thereby the thermal load rapidly increases such that the refrigeration unit continuously operates on full power. Additionally, a number of thermal sources such as cooking stoves exist in the kitchen and thereby the ambient temperature easily rises. In these circumstances, the condenser is prone to degrade in its heat discharge.
The applicant actually measured the ambient temperature concerning an industrial refrigerator installed in the kitchen of a restaurant as an example. Many cooking stoves in the kitchen were ignited during a busy time, and then the temperature in the circumference of the condenser (generally disposed on the top of the heat insulating box) of the refrigeration unit immediately began to rise. It reached about 45° C. on average, especially in the summer, and temporarily reached 50-60° C.
In such an environment, the pressure of the high-pressure side of the refrigerant circuit is projected to be extremely high, and therefore the protective operation for the refrigeration unit is performed at short intervals. Consequently, the temperature inside the refrigerator rises, and food or the like inside the refrigerator may lower in quality.
To address the above circumstances, the refrigeration unit may be designed to be tolerant of high pressure by employing high-pressure-tolerant parts as components thereof. However, such parts are very expensive, and it costs a great deal to do a test for proving design changes in this case.